This invention is in the field of adaptive imaging telescopes, and in particular such telescopes that compensate images viewed through atmospheric turbulence.
The atmosphere has a substantially uniform effect on a portion of a wavefront impinging on a small aperture, say 4 inches or less in diameter, at visible wavelengths. The impinged wavefronts can be decomposed into phase effects, and amplitude or scintillation effects. The phase effects are of major importance causing nearly a spatially uniform but time-varying random advance or retardation of the wavefront which can easily be as large as several times 2.pi. radians.
In large telescopes which may be considered as compounded telescopes due to superposition of the above small apertures, the first effect mentioned is overwhelmingly troublesome. More specifically, with a point image, the subapertures may be paired, and it may be shown that each pair of subapertures generates its own characteristic Fourier component fringe pattern in the image plane. In the distortionless propagation case, all of these fringe patterns add constructively at the image center and effectively add more and more destructively at positions in the image plane progressively removed from the center. Since the first mentioned effect can change either the position of the fringes or the phases of the optical carrier, which is sptially modulated by them, it can readily happen that the desired constructive interference on axis, and destructive interference off axis, is not obtained. Consequently, peak image intensity is reduced and the image is not narrowed by the superposition of fringe patterns, or at least it is not narrowed to the full potential of the system. Thus, images are smeared out by these atmospheric effects and imaging through long atmospheric paths with apertures exceeding 4 inches is usually limited by this phenomenon rather than by the optics of the imaging system.
Prior art systems have employed several restoration schemes. Some classificational schemes are post-film-recording or restoration systems, and pre-film-recording or pre-correction systems. By far, the largest number of schemes have been of the post-film-recording category and, as one conseqeunce, have not been real-time systems. Of the pre-film-recording techniques, holographic methods have been the most widely explored, and again these have not been in real time. Many suggested techniques, including holography have required a reference, that is a known, ideally a small source, to one side of the unknown source object. In most practical cases an ideal reference natural source is simply not available.